The present invention relates to the quantitative analysis of different anions or cations in a single system. Reversed-phase liquid chromatography (RPLC) is widely used as a mode of separation in high performance liquid chromatography (HPLC). In RPLC, the mobile phase is more polar than the stationary phase, the reverse being true in conventional chromatography performed prior to development of RPLC. Chemically bonded hydrocarbon chains (alkyl groups) attached to silica substrates are one common form of stationary phase. The mode of formation of such stationary phases and suitable techniques for performing RPLC are well known as set out, for example, in N. H. C. Cooke and K. Olsen, Am. Lab., 45 (August, 1979). One technique of RPLC has gained sufficient popularity to be called Reversed Phase Ion Pair Chromatography. In this technique, a salt is added to the mobile phase to improve the chromatographic properties. While there is some conflict in the theory of separation, the experimental techniques described in this paper are commonly employed. Specifically, the sample is directed in an aqueous polar mobile phase, commonly including a lower alcohol, acetonitrile or other water miscible organic solvent, together with a counter ion, typically tetrabutyl ammonium ion (TBA), for anion analysis. In one theory, hydrophobic ion pairs are formed which are relatively nonpolar and so are differentially retarded by the column. In another theory, the counter ion, e.g., TBA, is adsorbed to the surface to form a reversible ion exchange site on the stationary phase. This technique is employed primarily for the chromatographic separation and analysis of organic acids. However, typical detection techniques, such as ultraviolet detection, are unsuitable for the analysis of inorganics separated by RPLC. Also, for organic molecules, such as surfactants, the limit of ultraviolet detection does not provide for high sensitivity from lack of a strong chromaphore.
Another chromatographic system known as ion chromatography has been utilized in the quantitation of organic and/or inorganic anions and/or cations in aqueous sample solutions. In this technique, chromatographic separation is performed on low capacity ion exchange separating resin column or columns. Then the eluent is directed through a high capacity ion exchange resin suppressor column which converts the eluent from a conducting form to a non-conducting form and thereby reduces the background conductivity of the chromatographic system. The ions to be analyzed are eluted from the suppressor column and form highly conductive species which are passed through a conductivity cell and quantitated on the basis of conductivity. This technique is well suited to ionic species eluting from the suppressor column in a form which has a dissociation constant of greater than 10.sup.-7. Molecules with dissociation constants less than this are not detectable by conductivity at chromatographic concentration levels.
One limitation to ion chromatography is that the separating resin must be of a conventional permanent ion exchange site containing type. This substantially fixes both the ion exchange capacity and selectivity of the separating column since the ion exchange groups are chemically bonded to the substrate resin. Thus, for a given column and resin, modification of the chromatographic resolution would require chemical modification of the resin, such as by changing of the ion exchange groups by substituting a different type of resin, a time consuming and costly operation. The capacity of the separating resin must be small so that relatively low ionic strength eluents can be used to maximize suppressor column lifetimes. The resolution of highly ionized ionic species in accordance with this technique is set out, e.g., in Small et al. U.S. Pat. No. 3,920,397.